Method and apparatus for making and using an improved fiducial for an integrated circuit

ABSTRACT

In one embodiment, the present invention includes a method including the following acts. A light source is scanned over a surface of an integrated circuit. A photo-induced current is measured from a fiducial in the integrated circuit. The current is correlated to a position of the light source as the scanning progresses.

The present patent application is a Divisional of prior application Ser.No. 09/451,631, filed Nov. 30, 1999, entitled “Method And Apparatus ForMaking And Using An Improved Fiducial For An Integrated Circuit.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to integrated circuits and morespecifically to integrated circuit processing, design, and debugging.

2. Description of the Related Art

Prior art fiducials have typically been produced with a single layer ormultiple layers of metals deposited on a semiconductor substrate in acharacteristic shape, such as a cross or plus-sign (‘+’) shape, or somesimilar but recognizable shape. By using a recognizable shape, thesefiducials have been constrained to be large patterns which provedistinctive when viewed by the people who use them for navigating on asemiconductor integrated circuit. A fiducial in the prior art wouldoften consume a square-shaped space on the integrated circuit 150 μm ona side, which could only be used for the fiducial, not for any activecircuitry. As a result, valuable resources on the integrated circuitwould be unavailable in that region.

FIG. 1 illustrates one prior art scheme for placement of fiducials.Package 1 10 contains integrated circuit 130. Package 110 also have fourpackage fiducials 120 located on the outside of package 110, which areused by someone who needs to locate a specific portion of integratedcircuit 130. After locating and aligning to a first package fiducial120, a portion of the package 110 may be removed to expose integratedcircuit 130. Each of four fiducials 140 are incorporated into integratedcircuit 130. Upon aligning to a first fiducial 140, a person may thennavigate over the integrated circuit 130 by looking at a layout diagramof integrated circuit 130 which shows the location of the fiducials 140relative to the circuitry incorporated in integrated circuit 130.

As will be appreciated, positioning the fiducials such as fiducials 140proves difficult due to constraints on available space on integratedcircuit 130. In the case of a fiducial consuming a square of space 150μm on a side, four such squares must be reserved in the area availableon integrated circuit 130, and no other signals may be routed in thosereserved areas. In some prior art processes, the fiducial must bedeposited in every significant layer of deposition in the semiconductorfabrication process.

Furthermore, even in situations in which automated alignment equipmentis used, such equipment must use an optical system for locating thefiducials. Whether human, mechanical, or some combination of human andmechanical, the optical systems are limited by their inability toresolve images below a certain size (length or area) threshold onsemiconductor devices. This limitation leads to a limitation on the sizeof fiducials used for alignment when using optical alignment systems,thus leading to the 150 μm length of prior art fiducials. It will beappreciated that even though an optical system may be capable ofresolving features much smaller than the overall size of a fiducial,that the need for a distinctive shape of the fiducial leads to afiducial much larger than the size of the smallest feature an opticalsystem may resolve.

SUMMARY OF THE INVENTION

In one embodiment, the invention includes a method. The method includesscanning a light source over a surface of an integrated circuit. Themethod also includes measuring a photo-induced current from a fiducialin the integrated circuit. The method also includes correlating thecurrent to a position of the light source as the scanning progresses.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures.

FIG. 1 illustrates a prior art block diagram of fiducials on a packagedintegrated circuit.

FIG. 2 illustrates an embodiment of a fiducial.

FIG. 3 illustrates an alternate embodiment of a fiducial.

FIG. 4 illustrates a side view of a fiducial in a packaged integratedcircuit as it may be scanned by a light beam.

FIG. 5 illustrates a current response of one embodiment of a fiducialwhen a light beam scans over the area of the fiducial.

FIG. 6 illustrates a configuration of fiducials and bond pads on anintegrated circuit.

FIG. 7 illustrates an alternative configuration of fiducials on anintegrated circuit.

FIG. 8 provides a block diagram of a method of using a fiducial.

FIG. 9 provides a block diagram of a method of making a fiducial.

DETAILED DESCRIPTION

A method and apparatus for making and using an improved fiducial for anintegrated circuit is described. In the following description, forpurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the invention. It will beapparent, however, to one skilled in the art that the invention can bepracticed without these specific details. In other instances, structuresand devices are shown in block diagram form in order to avoid obscuringthe invention.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, but the various embodiments may not be construed asmutually exclusive, either.

Illustrated in FIG. 2 is one embodiment of a fiducial suitable for usein finding portions of an integrated circuit. Substrate 210, in oneembodiment, is a silicon substrate with a light p-type doping. Implantarea 220 is a portion of the silicon substrate in which an n-type dopanthas been introduced to produce a local n-type well or field. Thejunction between substrate 210 and implant area 220 forms a pn junction.Substrate 210 may be thought of as having a dopant of a first type andImplant area 220 may be thought of as having a dopant of a second type.Connected or coupled to implant area 220 is contact 230, and coupled tocontact 230 is conductor 240. Not shown is the coupling of conductor 240to a bond pad or other portion of the silicon substrate which may beused to observe the voltage level of conductor 240 or current flowingthrough conductor 240.

Turning to FIG. 3, another embodiment of a fiducial is illustrated.Again, substrate 310 is a p-type silicon substrate, and implant area 315is an area doped with an n-type dopant, thus forming a pn junction.Furthermore, contact 320 is connected to implant area 315, and contact320 couples to conductors 325. Additionally, alignment features 330 arealso included.

Turning to FIG. 4, use of a fiducial is illustrated. Substrate 410 is aflip-chip or C4-mounted integrated circuit (C4 is an abbreviation for'Controlled Collapse Chip Connection). At a variety of bond pads onSubstrate 410, solder balls 480 are connected to substrate 410.Connected to solder balls 480 is package 420. Conductor 450 connects orcouples a first solder ball 480 to lead 460, which in turn connects orcouples to sensor 490. Sensor 490 may alternately be a voltage orcurrent sensor. Within Substrate 410, conductor 440 couples the firstsolder ball 480 to fiducial 430. In one embodiment, fiducial 430encompasses the implant area 220 and contact 230, while conductor 240may be conductor 440 and substrate 210 may be substrate 410. Using lightbeam 465 as focused by lens 470, light is scanned across substrate 410.

Silicon and most other semiconductors are relatively transparent tolight in the infrared part of the spectrum, but experimentationindicates that light in the spectrum from the near-visible infraredthrough the visible spectrum is suitable for use with the fiducial. Inone experiment, light at the 514 nm wavelength was found to be usefulwith the fiducial, and experimentation also indicates that light in theinfrared spectrum far removed from the visible spectrum is less usefulwith the fiducial. Light scanned across the substrate may come from suchlight sources as a continuous light beam, a strobed light beam, apolychromatic or a monochromatic source, among others.

As a result, light beam 465 or the photons embodied therein may beprojected into substrate 410. When light beam 465 interacts withfiducial 430, a photo-induced current results, which may be measured bysensor 490. Thus, an observer may scan the light beam 465 across theback side surface of substrate 410, and note the location of fiducials430 based on the current indicated by sensor 490 when the light beam 465is at projected at various locations on substrate 410. FIG. 5illustrates a typical response curve for a sensor such as sensor 490, inwhich the measured current I is plotted against proximity to fiducial430. In the region 510 and region 530, the light beam 465 is far enoughfrom fiducial 430 that essentially no photo-induced current is measuredby sensor 490. However, in region 520, light beam 465 is close enough tofiducial 430 that a photo-induced current is measured by sensor 490,with that current peaking when light beam 465 fully overlaps fiducial430.

A fiducial designed to be used in this manner may be designed to be muchsmaller than the prior art fiducials illustrated in FIG. 1. Experimentswith one embodiment of a fiducial as described have shown that afiducial in which the implant area such as implant area 315 or implantarea 220 is formed as a square of length 15 μm per side allows forfinely tuned navigation to other features on the integrated circuit aspredicted by layout diagrams corresponding to the manufacturingprocesses for the integrated circuit. In one instance, navigation within0.1 μm of the actual location of other features on the die wasdemonstrated, based solely on navigating from the fiducials. Moreover,such experiments indicate that the size of such a fiducial may befurther reduced, as photo-induced currents on the order of 1 mA may beproduced with a 15 μm square as described above, while currents ofsignificantly lower magnitude may be detected reliably in suchsituations. Also, it will be appreciated that varying the amount ofenergy carried by the light beam used for scanning, such as light beam465, may result in variations in the magnitude of the photo-inducedcurrent, such that smaller fiducials may be used with light sources ofhigher intensity or power.

Turning to FIG. 6, one embodiment of an integrated circuit containingfiducials such as those described in relation to FIGS. 2 and 3 isillustrated. Bond pads A are spaced at regular intervals throughout thesemiconductor substrate. Likewise, fiducials B are also spaced atregular intervals throughout the substrate. One bond pad, bond pad C, iselectrically coupled to all of the fiducials B, such that aphoto-induced current from any fiducial may be measured by a sensorcoupled to bond pad C. It will be appreciated that FIG. 6 illustrates ablock diagram, and that locations and connections therein are not scaledrelative to each other. For instance, Bond pads A are typicallysquareshaped in conventional semiconductor technologies, but may beformed in any shape desired. Likewise, the relative sizes of Bond pads Aand C and fiducials B are not illustrated, as fiducials B may be sizedto be significantly smaller than Bond pads A and C. It will beappreciated that more fiducials B may be positioned on a substrate thanthe nine illustrated in FIG. 6, particularly since the fiducials B maybe made small enough to fit between other circuitry embodied in anintegrated circuit.

Alternatively, FIG. 7 illustrates another embodiment of an integratedcircuit containing fiducials as described in relation to FIGS. 2 and 3.Bond Pads P are located on the perimeter of the integrated circuit.Fiducials 710 are distributed throughout the surface of the substrate inthe integrated circuit. In one embodiment, a first set of four fiducials710 are coupled together to bond pad P1, a second set of four fiducials710 are coupled through conductor 720 to bond pad P2, and a third set offour fiducials 710 are coupled through conductor 730 to bond pad P3.Thus, some indication of which fiducial is being scanned by a light beammay be derived from analysis of which bond pad P is receiving thephoto-induced current. In an alternate embodiment, the second set offiducials is not coupled to bond pad P2 through conductor 720, but tothe first set of fiducials through conductor 750. Likewise, the thirdset of fiducials is coupled to the first set of fiducials throughconductor 740. Thus, all of the fiducials are coupled to common bond padP1, and one bond pad may be monitored to observe photoinduced currentfrom any of the fiducials. This bond pad may be dedicated for use onlyin conjunction with the fiducials, or may be used for other purposeswhen the circuit is in use, such as power supply (Vcc) for example.

Turning to FIG. 8, a flow diagram of one embodiment of a method of usingthe fiducials described in relation to FIGS. 2 and 3 is illustrated.Initially, the substrate of the semiconductor is thinned globally inGlobal Thin 810, making it more transparent to light. Scan 820encompasses scanning a light source or optical source over a surface,and measuring the photo-induced currents produced when the light passesover the fiducials, thus deriving a rough map of a portion or of theentire substrate. Next, the substrate is thinned again in the areas ofinterest, either at the fiducials or at locations derived from theobserved locations of the fiducials at Local Thin 830. At this point,actual debugging of the integrated circuit or other profiling of thesubstrate.

Turning to FIG. 9, a flow diagram of how a fiducial may be made in oneembodiment is provided. The method of making the fiducial may bedescribed with reference also to FIG. 2. Initially, Providing aSubstrate 910 occurs, in which a substrate such as the substrate 210 ofFIG. 2 is provided. Following that, a pn junction is created, atCreating a pn Junction 920. One example of creating a pn junction isillustrated by implanting a n-type dopant into a p-type doped substrate,such as may occur to create implant area 220. Next, Forming a Contact930 occurs, in which a contact to the pn junction such as contact 230 isformed. Next, Coupling a Conductor 940 occurs, in which a conductor suchas conductor 240 is couple to the contact, such as contact 230, therebyallowing for an electrical signal to flow to and from the pn junction.Finally, Coupling a Bond Pad 950 occurs, in which the conductor iscouple to a bond pad, thereby allowing for probing of the electricalsignals flowing to and from the pn junction. It will be appreciated thatthe method may encompass more or less than exactly what is outlinedhere. For instance, probing of the pn junction may occur at the contact,thus making the conductor and bond pad less important or unnecessary.Furthermore, the method may also encompass bonding out the bond pad to awire in a package, thus allowing for access to the signals from outsidea packaged integrated circuit.

In the foregoing detailed description, the method and apparatus of thepresent invention has been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. For example, theexemplary embodiments have been discussed with respect to a pn junctionformed in silicon, but a similar junction formed in Gallium-Arsenide orother semiconducting materials may be used to form a fiducial in such asemiconductor within the spirit and scope of the present invention.Likewise, it will be appreciate that using a fiducial on a integratedcircuit which is wire-bonded or otherwise connected to a package thatdoes not use C4 technology may be accomplished within the spirit andscope of the present invention. The present specification and figuresare accordingly to be regarded as illustrative rather than restrictive.

What is claimed is:
 1. An apparatus comprising: a fiducial in anintegrated circuit including: a substrate with a first dopantconcentration of a first type; a doped portion of the substrate having asecond dopant concentration of a second type, the second dopantconcentration greater than the first dopant concentration, the secondtype different from the first type, such that a junction of the dopedportion of the substrate with the substrate forms a pn junction; and acontact coupled to the doped portion of the substrate, the contactaccessible electrically outside of the integrated circuit.
 2. Theapparatus of claim 1 wherein: the substrate is a silicon substrate. 3.The apparatus of claim 2 wherein: the first type is p-type and thesecond type is n-type.
 4. The apparatus of claim 3 further comprising: abond pad, the bond pad coupled to the contact; a package, the packageenclosing the substrate, the package having a plurality of leads, a leadof the plurality of leads coupled to the bond pad.
 5. The apparatus ofclaim 4 wherein: the bond pad is connected both to the fiducial and toother circuitry of the integrated circuit.
 6. The apparatus of claim 3wherein: the doped portion is shaped as a square having a sideapproximately 15 μm long.
 7. The apparatus of claim 3 wherein: the dopedportion is shaped as a square.
 8. The apparatus of claim 2 wherein: thefirst type is n-type and the second type is p-type.
 9. The apparatus ofclaim 8 further comprising: a bond pad, the bond pad coupled to thecontact; a package, the package enclosing the substrate, the packagehaving a plurality of leads, a first lead of the plurality of leadscoupled to the bond pad.
 10. The apparatus of claim 9 furthercomprising: a sensor coupled to the first lead, the sensor measuring aphoto-induced current from the fiducial when exposure of the fiducial toa light results in the photo-induced current.